1. Field of the Invention
The present invention pertains to high voltage electrical equipment enclosed in a metallic housing and having insulating bushings for high voltage electrical connections extending through the housing.
2. Description of the Related Art
Many types of high voltage electrical equipment in use today have outer metallic structures which serve as a rugged shipping container and which protect the internal components of the electrical device. Due to inefficiencies of practical electrical components, heat is generated within the metallic structure. Accordingly, some means of providing cooling for components internal to the structure must be provided. Of interest here are liquid-filled and gas-filled electrical devices in which the electrical components are totally immersed in a fluid (herein, a non-air fluid) comprising the electrically insulating heat conducting medium. Fluid cooling media are preferred over air-filled equipment, since fluid-cooled devices can be made substantially smaller. It has been found convenient to fashion the outer metallic container as a pressure vessel to provide adequate leak-free containment of the dielectric cooling fluid disposed therein.
High voltage electrical devices, particularly those commonly employed in the electrical power industry, have average life expectancies of several decades. When properly installed, electrical devices of the type referred to herein typically require little or no maintenance during their operating life. Accordingly, in order to prevent unnecessarily premature maintenance operations, an outer metallic structure must be leak-free. This is particularly critical for fluid-filled electrical equipment in which the outer metallic structure is filled with a dielectric gas such as sulphur hexafluoride. In the past, considerable attention has been paid to gaskets of removable doors and the like, and to the leak-free integrity of welding operations used to construct the outer metal closures.
Practical electrical devices must have provision for connecting the internal components thereof to external electrical systems, such as a power distribution system, for example. Electrical components operated at higher voltages require special design considerations for high voltage conductors which pass through a ground plane, such as a side wall of a metal closure. In order to reduce electrical stress at points of close spacing between the high voltage electrical conductor and the grounded closure, electrical insulating bushings are employed to provide an increased dielectric strength. The bushings, typically made of porcelain or an epoxy material, have significant mass and require substantial physical support from the metal closure so as to withstand mechanical stresses due to the weight of the bushing and also the weight of the relatively massive electrical conductors which are fastened to the ends of the bushing so as to receive mechanical support therefrom.
Two modes of mechanically securing a bushing to a metal closure are in popular use today. In a first mode, the bushing is bolted to the metal closure and gasket rings are compressed between the bushing and the wall of the metal closure to provide a fluid-tight seal. In a second type of bushing construction, a metallic flange, is wedded to the bushing insulation body. For example, one type of bushing in use today has a plug-cast epoxy body which is molded around the radially interior portion of an annular metal mounting ring, resembling an annular disk. The radially exterior portions of the mounting ring are left uncovered by the bushing body and provide a convenient flange for welding to the wall of a metal closure. However, as will be appreciated by those skilled in the art, such welding is performed at a relatively close spacing to the molded epoxy body and precautions must be observed to prevent damage to the bushing body caused by the heat of the welding operation. Such welded bushing flanges provide an alternative method to bolted gaskets, and are preferred in certain types of electrical installations today, such as those applications having a gas composition such as sulfur hexafluoride as an insulating medium.
Although precautions are taken during welding, such as providing a heat sink at the radially inner portions of the welding collar, immediately adjacent the dielectric bushing body, deficiencies in the fluid-tight seals of such welded bushing constructions have been observed in long term tests. Even if elaborate precautions are taken during the welding operation, it is possible that the fluid-tight seal between the dielectric bushing body and the metallic welding flange will be compromised during construction of the device. Due to the very long life expectancy of electrical equipment and the relatively high pressures of the dielectric gases which must be contained by the bushings as well as the metal closure, even relatively minor, so called "fine" leaks assume a critical importance. For the purposes herein, a "fine" leak is on of the order of one millionth of a cubic centimeter- atmosphere per second. The epoxy - metal interface of such welded bushings, it is observed, frequently exhibits insidious fine leaks. Significant resources are expended today in testing and returning to the manufacturing facility, bushings found to have fine leaks. In order to improve the economies of operation, it is desirable to provide a bushing which is inherently reliable in its ability to remain leak-free even with respect to fine leaks, over relatively long periods of time.
Occasionally, a number of electrical insulating bushings must be welded to the outer surface of a metal closure, in a high density, closely spaced array. The bushings, when placed close to one another, present difficulties to a manufacturer of electrical equipment in that it is difficult to orient the welding equipment in an optimum manner. For example, bushings having welding flanges formed therewith are inserted through a closure wall, with the welding flange being pressed against portions of the closure wall surrounding a bushing-receiving aperture. Next, the outer periphery of the welding flange is welded to the outer surface of the metal closure. It is generally desirable to orient a welding electrode or the like source of welding heat at a 45.degree. angle to the surface of the metal closure. The welding equipment is preferably revolved in a circle concentric with that of the bushing. However, when the bushings are closely spaced together, and extend significant distances outwardly beyond the wall of the metal closure, adjacent bushings tend to obstruct the welding equipment, and the desired 45.degree. orientation is not possible. This results in time consuming adjustments necessary to complete the welding operation.